집속형 레이저 유도초음파에 의한 결함검출

김홍준(Hongjoon Kim),정지홍(Jihong Jung),하욥(Job Ha),장경영(Kyungyoung Jhang) 대한기계학회 2003 대한기계학회 춘추학술대회 Vol.2003 No.4

Arc-shaped line array slits have been used for the laser generation of focused Lamb waves. The spatially<br/> expanded Nd:YAG pulse laser was illuminated through the arc-shaped line array slit on the surface of a<br/> sample plate to generate the Lamb waves of the same pattern as the slit. Then the generated Lamb waves were<br/> focused at the point of which distance from the slit position is dependent on the curvature of slit arc. The<br/> proposed method showed better spatial resolution than the conventional linear array slit in the detection of<br/> laser machined linear defect and drill machined circular defect on aluminum plates of 1mm thickness.

복수의 진동 모터 사이의 전환적 사용을 통한 빠른 연속 진동 피드백 생성 및 사용자 경험 개선

김홍준(Hongjoon Kim),김정현(G. JoungHyun Kim) 한국HCI학회 2014 한국HCI학회 학술대회 Vol.2014 No.2

현대 모바일 기기에서 진동 모터는 많은 장비에서 필수적인 옵션으로 많은 장비에 장착되어 있다. 하지만 거의 1 개만 장착되어 사용 중이고, 1 개만으로는 다양한 효과를 주기 힘든 한계가 있다. 특히, 빠른 이벤트가 연속적으로 들어올 때, 1 개의 진동모터로는 새로운 진동 이벤트 처리가 불가능한 한계가 존재한다. 예를 들어, 인간은 100~200ms 단위의 (혹은 그 이하) 촉각 이벤트를 구분 하거나 인터액션이 가능하지만, 진동 모터의 주파수/세기에 대한 Switching 반응 time 은 이보다 대개 길다. 따라서, 본 논문에서는 2 개의 진동 모터를 전환적으로 이용하여 빠른 연속적 촉각 피드백을 사용자가 느낄 수 있도록 하고 이를 검증하는 실험을 진행 하였다. 그 결과 진동 모터 1 개를 사용할 때 보다 연속적인 개별 피드백을 좀 더 잘 인지 할 수 있을 뿐 아니라 전체적인 체감효과나 사용자 경험이 개선 되는 것을 확인 하였다. Modern hand-held devices are often equipped with a single vibration motor to provide simple on-off type of vibro-tactile feedback. There is an inherent limitation with the single vibrator scheme in terms of providing more enriched feedback. For example, when there is a need to generate feedback for fast successive events, the single vibrator scheme often falls short due to its slow switching and response time (> 500ms) to frequency or amplitude change. On the other hand, the temporal resolution for humans in perceiving tactile events is in the order of 100s of milliseconds or even lower. Humans are also capable of generating interactive events at the rate of 100’s of milliseconds. Therefore, in this paper, we propose to use two or more vibrators and switch between/among them to realize fast and clear provision of individually separated tactile event feedback. We carried out a validation experiment and confirmed the projected effects and also observed overall improved user experience as well.

메타 물질을 이용한 초소형, 광대역 90° 커플러

김홍준(Hongjoon Kim) 한국전자파학회 2012 한국전자파학회논문지 Vol.23 No.7

메타 물질의 한 형태인 LHTL(Left-Handed Transmission Line)과 기존의 전송 선로 형태인 RHTL(Right-Handed Transmission Line)을 이용하여 광대역 I-Q 벡터 신호 생성을 위한 90° 커플러를 설계, 제작하고 측정을 하였다. LHTL과 RHTL 모두 커패시터와 인덕터를 이용하여 합성 전송 선로 형태로 구성함으로써, 그 크기를 최소화 하였다. 또한, 제안된 커플러 제작에 필요한 Wilkinson 전력 분배기를 합성 RHTL을 이용하여 간단하게 구현함으로써 전체 회로의 크기를 11 ㎜×12 ㎜로 만들 수 있었다. 주파수 범위 0.8~1.25 ㎓에 대해 출력의 위상 차이가 90°±5°를 유지함으로써 광대역 90° 커플러를 작은 크기로 만들 수 있었다. 동 주파수 범위에 대해 삽입 손실을 1.6 ㏈ 이하로, 반사 손실을 10.1 ㏈ 이상으로 유지 가능했다. 필자가 아는 한 이는 그 주파수 대에서 가장 작은 광대역 90° 커플러이며, MMIC(Monolithic Microwave Integrated Circuit)로 만들 경우 그 크기를 훨씬 더 줄일 수 있을 것이다. By using LHTL(Left-Handed Transmission Line) which is a form of a metamaterial and conventional RHTL (Right-Handed Transmission Line), we designed, fabricated and tested a broadband 90° coupler which is a basic circuit for I-Q vector signal generation. Synthetic LHTL and RHTL were implemented with capacitors and inductors only, that the size is minimized. Also, by implementing a Wilkinson power divider which is required for the suggested circuit using a synthetic RHTL, the size of whole circuit is only 11 ㎜×12 ㎜. For the frequency range 0.8~1.25 ㎓, the phase difference at the outputs maintained 90°±5° and thus, a broadband 90° coupler could be made in a compact form. for the same frequency range, the insertion loss is less than 1.6 ㏈ and return loss is more than 10.1 dB. To the best of our knowledge, this is the smallest and broadband 90° coupler for the frequency range and if the circuit is made with MMIC(Monolithic Microwave Integrated Circuit) technology, the size will be reduced much further.

7 T MRI에서의 다양한 RF 코일에 대한 B₁⁺ 비교

김홍준(Hongjoon Kim),유형석(Hyoungsuk Yoo) 한국전자파학회 2012 한국전자파학회논문지 Vol.23 No.8

정 자장(B?)의 세기가 7T(Tesla) 고자기장 MRI(Magnetic Resonance Imaging) 시스템은 정 자장의 세기가 1.5 T 또는 3 T MRI 시스템에 비하여 인가된 RF(Radio Frequency) 필드의 높은 불균질성을 보여준다. 이러한 문제점를 극복하기 위한 방법으로써, 관심 영역에서의 RF 자장이 균일하도록 RF 코일의 각 요소로 들어가는 최적화된 전류의 크기와 위상 값을 얻기 위해서 convex 최적화 방법이 사용된다. 이러한 방법을 7 T MRI 시스템에 다양한 RF 코일에 적용하여 각각의 B₁? 필드 값을 비교하여 그 성능을 파악하였다. This paper demonstrates the use of the convex optimization to localize the transverse magnetic B₁?field in regions of interest for recently proposed multi-sectioned alternating impedance coils and the traditional transmission line coil. An approach based on different axial slices to identical RF coils except upper stripline structure is investigated. Electromagnetic simulation results are compared for RF coils and discussed in detail at 7.0 T.

중심주파수에서 원하는 위상변위가 가능한 대역통과 필터

김홍준(Hongjoon Kim) 대한전기학회 2012 전기학회논문지 Vol.61 No.7

By cascading RHTL (Right-Handed Transmission Line) and LHTL (Left-Handed Transmission Line), we fabricated a BPF (Band Pass Filter) in which the phase propagation at the pass band center frequency is fixed as we want. We utilized a positive phase propagation of a RHTL which is a form of LPF (Low Pass Filter) and negative phase propagation of LHTL which is a form of HPF (High Pass Filter). Therefore, if RHTL and LHTL are cascaded, a BPF can be constructed and the phase propagation inside the passband is decided by the number of RHTLs and LHTLs. In this paper, we provide a detailed theory related to it and proved the theory with an actual experiment. In the experiment, we fabricated two BPFs with similar passband. One with 90° phase shift and the other with -90° phase shift at the center of passband. The result of simulation and actual experiment agrees well. This proves the suggested theory is correct and feasible.

식물잎의 표면현상 및 마이크로-스케일에서의 마찰 특성

김홍준(Hongjoon Kim),아르빈드 싱(R. Arvind Singh),윤의성(Eui-Sung Yoon),공호성(Hosung Kong) 한국트라이볼로지학회 2005 한국트라이볼로지학회 학술대회 Vol.2005 No.11

Surfaces found in nature, including biological surfaces have been providing inspiration to modify/fabricate artificial surfaces as solutions for tribological applications. As an example, the concept of 'lotus-effect' has motivated tribologists world wide to modify/fabricate surfaces for enhanced tribological performance. These was done by creating nano/micro-scaIe asperities on various surfaces using ion beam milling and ion-beam assisted roughening. In order to understand the attributes of natural surfaces, which are inspirational to tribologists, we characterized the surface of two natural surfaces - Nelumbo nucifera (lotus) and Colocasia esculenta leaves. Further, we evaluated their micro-scale friction property, both in their fresh and dried conditions. The characterization of surfaces was conducted using a confocal microscope and SEM, which involved the evaluation of size and distribution of protuberances. The micro-scale friction property was evaluated using a ball-on-flat type micro-tribo tester, under reciprocating motion. A soda lime glass ball (2 ㎜ diameter) was used in these tests. Tests were conducted at the applied normal load of 3000μN, at a sliding speed of \ mm/sec for a scan length of 3 ㎜. All experiments were conducted at ambient temperature (24±1℃) and relative humidity (45±5%). It was observed that the friction behaviour of the natural surfaces was influenced by their surface characteristics (morphology and distribution of protuberances) and also by the condition (fresh or dried) in which they were tested.

다양한 MRI 시스템에서 사용가능한 의료용 리드선

김홍준(Hongjoon Kim),유형석(Hyoungsuk Yoo) 대한전기학회 2015 전기학회논문지 Vol.64 No.3

Radio Frequency (RF) coils in Magnetic Resonance Imaging (MRI) systems interact with a patient’s tissues, resulting in the absorption of RF energy by the tissues. The presence of an electrically conducting medical implant may concentrate the RF energy and causes tissue heating near the implant devices. Here we present a novel design for a medical lead to reduce this undesired heating. Specific Absorption Rate (SAR), an indicator of heating, was calculated. Remcom XFdtd software was used to calculate the peak SAR distribution (1g and 10 g) in a realistic model of the human body. The model contained a medical lead that was exposed to RF magnetic fields at 64 MHz (1.5 T MRI), 128 MHz (3 T MRI) and 300 MHz (7 T MRI) using a model of an MR birdcage body coil. Our results demonstrate that, our proposed design of adding nails to the medical lead can significantly reduce the SAR for different MRI systems.

레이더 센싱을 위한 빔 조정 기술과 센싱을 막기 위한 메타 반사판 기술

김홍준(Hongjoon Kim) 대한전기학회 2021 전기의 세계 Vol.70 No.12

레이더를 이용한 사물의 탐지 및 위치추적 기술은 현대 사회에서 군용뿐 아니라 민간용 등 많은 분야에서 활용 가능하다. 본 고에서는 전자기 센싱을 통한 위치추적 방법의 핵심이 되는 빔 조정(steering) 기술과 최근 대두되고있는 메타물질을 이용한 탐지 회피 기술에 대해 설명하고자 한다. Detection and ranging of an object using a RADAR is widely used for both military and civilian purpose. Here, the author explains the antenna beam steering technology which is a core in RADAR. In addition, a recent RADAR jamming technology using a Meta-surface is explained.

전자기파를 이용한 생체신호 인식과 위치판별

김홍준(Hongjoon Kim) 대한전기학회 2020 전기의 세계 Vol.69 No.3

전자기파를 이용한 비접촉식 생체신호의 인식 방법은 다른 소스 (빛, 열, (초)음파)를 이용한 방식 보다 환경의 변화에 영향을 적게 받으며, 제조 단가가 저렴한 장점이 있다. 하지만 인체의 유해성과 간섭등의 영향으로 전력의 크기가 제한되고, 잡음의 영향에 민감한 특성을 보이므로 신호대 잡음비 (SNR) 개선을 위한 많은 하드웨어적, 소프트웨어적 방법이 보고 되고있다. 본고에서는 저자의 최근연구 결과 중심으로 SNR향상을 위한 알고리듬과 위상배열 안테나를 이용한 위치추적 시스템에 대해 소개한다. Non-contacting human vital signal recognition using microwaves has many advantages compared to other methods (light, heat, sonic waves, etc) in terms of environmental variation and price. However, because it can be harmful to the human body and may interfere with other electronic devices, the power output is limited and the device can be sensitive to the noise. Therefore, many hardware and software methods are reported to minimize those effect and maximize the Signal-to-Noise Ratio (SNR). This paper demonstrates the author’s recent research results to increase SNR to acquire human vital signal and to localize such a signal using a phased antenna array.

식물잎의 표면형상 및 마이크로-스케일에서의 마찰 특성

김홍준(Hongjoon Kim),아르빈드 싱(R. Arvind Singh),김진석(Jinseok Kim),윤의성(Eui-Sung Yoon) 한국트라이볼로지학회 2006 한국윤활학회지(윤활학회지) Vol.22 No.5

Surfaces found in nature, including biological surfaces have been providing inspiration to modify/fabricate artificial surfaces as solutions for tribological applications. As an example, the concept of ‘lotus-effect’ has motivated tribologists world wide to modify/fabricate surfaces for enhanced tribological performance. These was done by creating nano/micro-scale asperities on various surfaces using ion beam milling and ion-beam assisted roughening. In order to understand the attributes of natural surfaces, which are inspirational to tribologists, we characterized the surface of two natural surfaces-Nelumbo nucifera (lotus) and Colocasia esculenta leaves. Further, we evaluated their micro-scale friction property, both in their fresh and dried conditions. The characterization of surfaces was conducted using a confocal microscope and SEM, which involved the evaluation of size and distribution of protuberances. The micro-scale friction property was evaluated using a ball-on-flat type micro-tribo tester, under reciprocating motion. A soda lime glass ball (2㎜ diameter) was used in these tests. Tests were conducted at the applied normal load of 3000 цN, at a sliding speed of 1 ㎜/sec for a scan length of 3㎜. All experiments were conducted at ambient temperature (24±1℃) and relative humidity (45±5%). It was observed that the friction behaviour of the natural surfaces was influenced by their surface characteristics (morphology and distribution of protuberances) and also by the condition (fresh or dried) in which they were tested.